1. Field of the Invention
The present invention relates to the signal transmission technologies, and it particularly relates to method and apparatus for transmitting signals in the packet format.
2. Description of the Related Art
In wireless communication, it is generally desired that the limited frequency resources be used effectively. Adaptive array antenna technology is one of the technologies that realize the effective utilization of frequency resources. In adaptive array antenna technology, the amplitude and phase of signals transmitted from and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. In other words, the apparatuses provided with adaptive array antennas change respectively the amplitudes and phases of signals received by a plurality of antennas, add up the thus changed received signals, and receive signals equivalent to the signals received by the antenna having the directional pattern corresponding to the variation in said amplitude and phase (hereinafter referred to as “weight”). And the signals are transmitted in a directional pattern of the antenna corresponding to the weight.
One example of processings to compute the weights in adaptive array antenna technology is a processing based on the MMSE (Minimum Mean Square Error) method. In the MMSE method, the Wiener solution is known to be the condition for an optimum weight value. Also known is a recurrence formula whose amount of calculation is smaller than that required to directly solve the Wiener solution. For such recurrence formula, adaptive algorithms, such as RLS (Recursive Least Squares) algorithm and LMS (Least Mean Squares) algorithm, are used. On the other hand, for the purpose of realizing a higher data transmission rate and improving the transmission quality, there is a case where data are subjected to multi-carrier modulation and the multi-carrier signals are transmitted (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Japanese Patent Application Laid-Open No. Hei10-210099.
There is a MIMO (Multiple Input Multiple Output) system as one of technologies by which to raise the transmission rate of data using the adaptive array antenna technology. A transmitting apparatus and a receiving apparatus in the MIMO system are each equipped with a plurality of antennas, and a channel suitable for each of the antennas is set. That is, the channel of up to the maximum number of antennas is set for the communication between the transmitting apparatus and the receiving apparatus, so as to improve the data transmission rate. Moreover, combining the MIMO system with a technique of transmitting multi-carrier signals results in a higher data transmission rate. On the other hand, the transmitted signals generally contain preambles that are known signals in order for the signals transmitted from the transmitting apparatus to be accurately received. In general, a preamble signal is specified by a fixed pattern. Nevertheless, if the pattern of a preamble signal varies taking into account the characteristics of radio channel and the packet utilization efficiency, it is possible to realize a wireless communication system which is flexible in terms of the characteristics of radio channel and the like.